This application claims the benefit of a Japanese Patent Application No. 2001-272601 filed Sep. 7, 2001, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium which is suited for high-density recording and capable of carrying out high-speed recording and reproduction, and to a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
Due to the developments in information processing technology, there are increased demands for high-density magnetic recording media. For example, for a hard disk, the magnetic recording media required to satisfy such demands should include such characteristics as low noise and improved thermal stability.
The recording density of longitudinal magnetic recording media, such as magnetic disks, has been increased considerably due to the reduction of medium noise and the development of magnetoresistive and high-sensitivity spin-valve heads. A typical magnetic recording medium is comprised of a substrate, an underlayer, a magnetic layer, and a protection layer which are successively stacked in this order. The underlayer is made of Cr or a Cr alloy, and the magnetic layer is made of a Co alloy.
Various methods have been proposed to reduce the medium noise. For example, Okamoto et al., xe2x80x9cRigid Disk Medium For 5 Gbit/in2 Recordingxe2x80x9d, AB-3, Intermag ""96 Digest, proposes decreasing the grain size and size distribution of the magnetic layer by reducing the magnetic layer thickness by the proper use of an underlayer made of CrMo. U.S. Pat. No. 5,693,426 proposes the use of an underlayer made of NiAl. Further, Hosoe et al., xe2x80x9cExperimental Study of Thermal Decay in High-Density Magnetic Recording Mediaxe2x80x9d, IEEE Trans. Magn. Vol. 33, 1528 (1997), for example, proposes the use of an underlayer made of CrTiB. The underlayers described above also promote c-axis orientation of the magnetic layer in a plane which increases the remanence magnetization and the thermal stability of the written bits. In addition, proposals have been made to reduce the thickness of the magnetic layer, to increase the resolution or to decrease the width of the transition between written bits. Furthermore, proposals have been made to decrease the exchange coupling between grains by promoting more Cr segregation in a magnetic layer which is made of the CoCr alloy.
However, as the grains of the magnetic layer become smaller and more magnetically isolated from each other, the written bits become unstable due to thermal activation and to demagnetizing fields which increase with linear density. Lu et al., xe2x80x9cThermal Instability at 10 Gbit/in2 Magnetic Recordingxe2x80x9d, IEEE Trans. Magn. Vol.30, 4230 (1994) demonstrated, by micromagnetic simulation, that exchange-decoupled grains having a diameter of 10 nm and the ratio KuV/kBTxcx9c60 in 400 kfci di-bits are susceptible to significant thermal decay, where Ku denotes the magnetic anisotropy constant, V denotes the average magnetic grain volume, kB denotes the Boltzmann constant, and T denotes the temperature. The ratio KuV/kBT is also referred to as a thermal stability factor.
It has been reported in Abarra et al., xe2x80x9cThermal Stability of Narrow Track Bits in a 5 Gbit/in2 Mediumxe2x80x9d, IEEE Trans. Magn. Vol. 33, 2995 (1997), that the presence of intergranular exchange interaction stabilizes written bits, as demonstrated by MFM studies of annealed 200 kfci bits on a 5 Gbit/in2 CoCrPtTa/CrMo medium. However, more grain decoupling is essential for recording densities of 20 Gbit/in2 or greater.
The obvious solution has been to increase the magnetic anisotropy of the magnetic layer. But unfortunately, the increased magnetic anisotropy places a great demand on the head write field which degrades the xe2x80x9coverwritexe2x80x9d performance, which is the ability to write over previously written data.
In addition, the coercivity of thermally unstable magnetic recording medium increases rapidly with decreasing switching time, as reported in He et al., xe2x80x9cHigh Speed Switching in Magnetic Recording Mediaxe2x80x9d, J. Magn. Magn. Mater. Vol. 155, 6 (1996), for magnetic tape media, and in J. H. Richter, xe2x80x9cDynamic Coervicity Effects in Thin Film Mediaxe2x80x9d, IEEE Trans. Magn. Vol. 34, 1540 (1997), for magnetic disk media. Consequently, adverse effects are introduced in the data rate, that is, how fast data can be written on the magnetic layer and the amount of head field required to reverse the magnetic grains.
On the other hand, another proposed method of improving the thermal stability increases the orientation ratio of the magnetic layer by appropriately texturing the substrate under the magnetic layer. For example, Akimoto et al., xe2x80x9cRelationship Between Magnetic Circumferential Orientation and Magnetic Thermal Stabilityxe2x80x9d, J. Magn. Magn. Mater. (1999), in press, report through micromagnetic simulation that the effective ratio KuV/kBT is enhanced by a slight increase in the orientation ratio. This further results in a weaker time dependence for the coercivity which improves the overwrite performance of the magnetic recording medium, as reported in Abarra et al., xe2x80x9cThe Effect of Orientation Ratio on the Dynamic Coercivity of Media for  greater than 15 Gbit/in2 Recordingxe2x80x9d, EB-02, Intermag ""99, Korea.
Furthermore, keepered magnetic recording media have been proposed for thermal stability improvement. The keeper layer is made up of a magnetically soft layer that is parallel to the magnetic layer. This soft layer can be disposed either above or below the magnetic layer. Oftentimes, a Cr isolation layer is interposed between the soft layer and the magnetic layer. The soft layer reduces the demagnetizing fields in the written bits on the magnetic layer. However, coupling the magnetic layer to a continuously-exchanged coupled soft layer defeats the purpose of decoupling the grains of the magnetic layer. As a result, the medium noise increases.
In order to improve the thermal stability and to reduce the medium noise, magnetic recording media and magnetic storage apparatuses have been proposed in U.S. patent application Ser. No. 09/425,788 filed Oct. 22, 1999, which is incorporated herein by reference, and in which the assignee is the same as the assignee of this application. This previously proposed magnetic recording medium is comprised of at least one exchange layer structure, and a magnetic layer formed on the exchange layer structure, wherein the exchange layer structure includes a ferromagnetic layer and a non-magnetic coupling layer provided on the ferromagnetic layer and under the magnetic layer, and the ferromagnetic layer and the magnetic layer have antiparallel magnetizations. According to this previously proposed magnetic recording medium, it is possible to improve the thermal stability of the written bits, reduce the medium noise, and realize a high-density recording having a high reliability without adversely affecting the performance of the magnetic recording medium.
In other words, in this previously proposed magnetic recording medium, the non-magnetic coupling layer (or the non-magnetic exchange layer) is interposed between the ferromagnetic layer that forms a first magnetic layer and the magnetic layer that forms a second magnetic layer. When the structure includes first and second magnetic layers having antiparallel magnetizations, the first and second magnetic layers mutually cancel portions of the magnetizations. Hence, it is possible to increase the effective grain size of the magnetic layer without substantially affecting the resolution. Therefore, from the point of view of the grain volume, it is possible to increase the apparent thickness of the magnetic layer so as to realize a magnetic recording medium having a good thermal stability.
Accordingly, this previously proposed magnetic recording medium employs a basic structure made up of the ferromagnetic layer (the first magnetic layer) and the magnetic layer (the second magnetic layer), so as to improve the thermal stability and to reduce the medium noise.
When an external recording magnetic field is applied to this previously proposed magnetic recording medium, the first and second magnetic layers first assume parallel magnetizations, and when the recording magnetic field decreases to zero (residual magnetization state) thereafter, the magnetization of the first magnetic layer is switched and becomes antiparallel to the magnetization of the second magnetic layer.
However, as the recording density and the signal transfer rate increase, it becomes necessary to also increase the recording and reproducing speed. For this reason, the need to wait for the switching of the magnetization to occur in the first magnetic layer after recording may interfere with the realization of high-speed recording and reproduction.
In other words, the first and second magnetic layers of this previously proposed magnetic recording medium assume antiparallel magnetizations in the residual magnetization state, and when the external recording magnetic field is applied in this state, the first and second magnetic layers assume parallel magnetizations. Then, when the recording magnetic field thereafter decreases to zero to assume the residual magnetization state once again, the magnetization of the first magnetic layer is switched to become antiparallel to the magnetization of the second magnetic layer. During this process, it is necessary to wait for the first magnetic layer to naturally make the magnetization switch.
But when the recording speed is increased and recording to an adjacent bit is made before the first magnetic layer makes the magnetization switch, the position of the bit which is to be recorded may shift due to a counter magnetic field from the bit in the parallel magnetization state. In this case, a non-linear transition shift (NLTS) deteriorates, and adversely affects the recording.
On the other hand, when measures are taken to reduce the time from recording to reproduction, an abnormal signal is generated to prevent normal reproduction if the reproduction is carried out before the first magnetic layer is switched to the antiparallel magnetization state from the parallel magnetization state.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium and magnetic storage apparatus, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a magnetic recording medium which has first and second magnetic layers with antiparallel magnetizations to realize improved thermal stability and reduced medium noise, and that is capable of carrying out magnetic recording and reproduction at a high speed, and to provide a magnetic storage apparatus which employs such a magnetic recording medium.
Still another object of the present invention is to provide a magnetic recording medium comprising a first magnetic layer, a second magnetic layer, and a non-magnetic coupling layer provided between the first and second magnetic layers so that the first and second magnetic layers are exchange-coupled and magnetizations of the first and second magnetic layers are antiparallel, where the first magnetic layer has an exchange coupling field Hex1 which is larger than respective coercivities Hc1 and Hc2 of the first and second magnetic layers. According to the magnetic recording medium of the present invention, the magnetizations of the first and second magnetic layers can be maintained antiparallel in a residual magnetization state, and it is possible to realize a high recording density and high-speed recording and reproduction.
A switching field Hsw* which switches the magnetization of the first magnetic layer to become parallel to the magnetization of the second magnetic layer may be set to a sum of the exchange coupling field Hex1 and the coercivity Hc1 of the first magnetic layer. In this case, it is possible to set a recording field within a range which does not reach the level of the switching field Hsw*, so that it is possible to positively realize a magnetic recording medium in which the magnetizations of the first and second magnetic layers are rotated while maintaining antiparallel magnetizations of the first and second magnetic layers.
A magnetization and thickness product t1Ms1 of the first magnetic layer is preferably smaller than a magnetization and thickness product t2Ms2 of the second magnetic layer, where t1 denotes a thickness of the first magnetic layer, Ms1 denotes a magnetization of the first magnetic layer, t2 denotes a thickness of the second magnetic layer, and Ms2 denotes a magnetization of the second magnetic layer. In this case, it is possible to increase the exchange coupling field Hex1 of the first magnetic layer having a small magnetization and thickness product t1Ms1, so that it is possible to more positively realize a magnetic recording medium in which the exchange coupling field Hex1 is larger than the coercivities Hc1 and Hc2 of the first and second magnetic layers.
The coercivity Hc1 of the first magnetic recording medium is preferably smaller than the coercivity Hc2 of the second magnetic recording medium. In this case, it is possible to determine a main-sub relationship of the first and second magnetic layers. In other words, it is possible to design a magnetic recording medium in which the second magnetic layer, which is set to have the large coercivity Hc2, is used as the main recording layer.
The magnetic recording medium may further comprise a coupling intensifying region, provided near the boundary of the non-magnetic coupling layer and at least one of the first and second magnetic layers, for intensifying the exchange coupling strength between the first and second magnetic layers. Further, the coupling intensifying region may be made of a material selected from a group consisting of Fe, Co, Ni and alloys thereof. With the coupling intensifying region, it is possible to obtain an exchange coupling field Hex which further increases the exchange coupling between the first and second magnetic layers.
A further object of the present invention is to provide a patterned medium comprising a recording surface, and a plurality of unit recording portions, provided on the recording surface, having boundaries which are separated among adjacent unit recording portions. Each of the plurality of unit recording portions preferably has a stacked structure comprising a first magnetic layer, a second magnetic layer, and a non-magnetic coupling layer provided between the first and second magnetic layers so that the first and second magnetic layers are exchange coupled and magnetizations of the first and second magnetic layers are antiparallel, where the first magnetic layer has an exchange coupling field Hex1 which is larger than respective coercivities Hc1 and Hc2 of the first and second magnetic layers. According to the patterned medium of the present invention, it is possible to realize a high recording density and high-speed recording and reproduction.
Another object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium, and at least one head for applying a field to the magnetic recording medium, where the magnetic recording medium comprises a first magnetic layer, a second magnetic layer, and a non-magnetic coupling layer provided between the first and second magnetic layers so that the first and second magnetic layers are exchange-coupled and magnetizations of the first and second magnetic layers are antiparallel, and the first magnetic layer has an exchange coupling field Hex1 which is larger than respective coercivities Hc1 and Hc2 of the first and second magnetic layers. According to the magnetic storage apparatus of the present invention, it is possible to realize high recording density and high-speed recording and reproduction.
The field from the head may be larger than a coercivity Hc2 of the second magnetic layer and smaller than a switching field Hsw* which switches the magnetization of the first magnetic layer to become parallel to the magnetization of the second magnetic layer. Moreover, the switching field Hsw* may be set to the sum of the exchange coupling field Hex1 and the coercivity Hc1 of the first magnetic layer. In these cases, it is possible to positively realize the high-speed recording.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.